The present invention relates generally to two-stroke internal combustion engines, and more specifically to a two-stroke internal combustion engine having a pair of opposed cylinders, each cylinder having a pair of opposed pistons.
The design and production of internal combustion engines for the automotive and light aircraft industries are well-developed fields of technology. To be commercially viable, any new engine configuration must, without sacrificing performance, provide significant improvements in the areas of energy and raw material conservation (especially the improvement of fuel consumption), environmental protection and pollution control, passenger safety and comfort, and competitive design and production methods that reduce cost and weight. An improvement in one of these areas at the expense of any other is commercially unacceptable.
A new engine configuration must be mechanically simple so that mechanical losses are inherently minimized, and must be well-suited to maximizing combustion efficiencies and reducing raw emissions. In particular, a new engine configuration should specifically address the most significant sources of friction in internal combustion engines to reduce mechanical losses; should have combustion chambers of a volume and design suitable for optimum combustion efficiency; and should be adaptable to utilizing advanced supercharging and fuel injection techniques.
A new engine configuration should be lighter in weight and preferably have a reduced height profile for improved installation suitability and passenger safety. For automotive applications, a reduced height profile would permit the engine to fit under the seat or floor area. For light aircraft applications, a short profile would permit installation of the engine directly within the wing, without the need for an engine cowling.
A new engine configuration should be dynamically balanced so as to minimize noise and vibration. Ideally, the smallest practical implementation of the engine, such as a two-cylinder version, should be fully balanced; larger engines could then be constructed by coupling smaller engines together. At low-load conditions, entire portions of the engine (and their associated mechanical losses) could then be decoupled without unbalancing the engine.
Despite the promise of external continuous combustion technologies such as Stirling engines or fuel cells with electric motors to eventually provide low-emission high-efficiency engines for automobiles and light aircraft, these technologies will not be viable alternatives to internal combustion engines in the foreseeable future due to their inherent disadvantages in weight, space, drivability, energy density and cost. The internal combustion piston engine will for many years continue to be the principal powerplant for these applications.
The four-stroke internal combustion engine currently predominates in the automotive market, with the four cylinder in-line configuration being common. The need for at least four cylinders to achieve a suitable rate of power stroke production dictates the size and shape of this engine, and therefore also greatly limits the designers' options on how the engine is placed within the vehicle. The small cylinders of these engines are typically not optimal for efficient combustion or the reduction of raw emissions. The four cylinder in-line configuration also has drawbacks with respect to passenger comfort, since there are significant unbalanced free-mass forces which result in high noise and vibration levels.
It has long been recognized by engine designers that two-stroke engines have a significant potential advantage over four-stroke engines in that each cylinder produces a power stroke during every crankshaft rotation, which should allow for an engine with half the number of cylinders when compared to a four-stroke engine having the same rate of power stroke production. Fewer cylinders would result in an engine less mechanically complex and less bulky. Two-stroke engines are also inherently less mechanically complex than four-stroke engines, in that the mechanisms for opening and closing intake and exhaust ports can be much simpler.
Two-stroke engines, however, have seen limited use because of several perceived drawbacks. Two-stroke engines have a disadvantage in mean effective pressure (i.e., poorer volumetric efficiency) over four-stroke engines because a significant portion of each stroke must be used for the removal of the combustion products of the preceding power stroke (scavenging) and the replenishment of the combustion air, and is therefore lost from the power stroke. Scavenging is also inherently problematic, particularly when the engine must operate over a wide range of speeds and load conditions. Two-stroke compression-ignition (Diesel) engines are known to have other drawbacks as well, including poor starting characteristics and high particulate emissions.
Modem supercharging and fuel injection methods can overcome many of the limitations previously associated with two-stroke engines, making a two cylinder two-stroke engine a viable alternative to a four cylinder four-stroke engine. A two cylinder two-stroke engine has the same ignition frequency as a four cylinder four-stroke engine. If the two-stroke engine provides a mean effective pressure ⅔rds that of the four-stroke, and the effective displacement volume of each cylinder of the two-stroke is increased to 3/2 that of the four-stroke, then the two engines should produce comparable power output The fewer but larger combustion chambers of the two-stroke would be a better configuration for improvement of combustion efficiency and reduction of raw emissions; the two-stroke could also dispense with the valves of the four-stroke engine, thus permitting greater flexibility in combustion chamber design.
Current production engines are also known to have significant sources of friction loss; increased engine efficiency can be achieved by reducing these friction losses. The largest sources of friction loss in current production automotive engines, accounting for approximately half of all friction losses, are the result of the lateral forces produced by the rotating connecting rods acting on the pistons, pushing them against the cylinder walls. The magnitudes of these losses are a function of the crankshaft throw, r, divided by the connecting rod length, l; the ratio is often designated λ (lambda). Decreasing λ, either by increasing the effective connecting rod length or decreasing the crankshaft throw, potentially yields the greatest overall reduction in friction loss.
The losses due to the contact of the pistons (or more correctly, the piston rings) with the cylinder walls are also a function of the mean velocity of the pistons with respect to the cylinder walls. If the pistons can be slowed down while maintaining the same power output, friction losses will be reduced.
Another significant source of friction loss in current production engines are the large forces acting on the crankshaft main bearings. A typical four cylinder inline engine has five crankshaft main bearings, which are necessary because there are literally tons of combustion force pushing down on the crankshaft; these forces must be transferred to the supporting structure of the engine. Both the crankshaft and the supporting structure of the engine must be designed with sufficient strength (and the corresponding weight) to accommodate these loads.